1. Field of the Invention
The invention relates to a method of providing an object of silicon steel with a heat-resistant, electrically insulating coating.
2. The Prior Art
It is known to provide sheet and strip of silicon steel, so-called electrical sheet and strip, with a heat-resistant electrically insulating coating in the form of a reaction product of silicon dioxide formed on the surface of the sheet and a metal oxide or hydroxide applied thereon. The application of such a protective layer on the surface of the sheet is usually performed by suspending an oxide or hydroxide of an earth alkali metal, especially of magnesium, in water and coating it on the sheet in the form of an even layer, after which the sheet material is heated to a temperature of about 1000.degree.C - 1350.degree.C in a hydrogen atmosphere for several hours. Simultaneously, the grain growth characteristic for the electric sheet takes place. The hydroxide, which is included in the suspension from the start, or which is formed from the oxide by reaction with water, liberates water during the heating of the sheet material which, at temperatures below that mentioned above, is capable of oxidizing silicon in the steel to silicon dioxide without the iron being oxidized at the same time. The oxide which is formed from the hydroxide during the liberation of water, or which has possibly been added from the start and has avoided hydration, reacts at about 1000.degree.C - 1350.degree.C with the silicon dioxide, thus forming a glass film on the surface of the sheet material. It is also possible to add particular additives to the water, which are capable of oxidizing silicon to silicon dioxide at higher temperatures. Of course, glass formation is performed also in this case. Any excess of the oxide of earth alkali metal, which has not reacted during the glass formation, acts as spacer material between adjacent layers of the sheet whether these occur as turns in a roll or as lamina in a stack, thus preventing the layers from sticking or sintering together.